1. Field of the Invention
The invention relates generally to the field of capacitors, and more particularly, to a system and method for producing electrodes for capacitors.
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an implantable cardioverter defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size and ability to withstand relatively high voltage. Conventionally, an electrolytic capacitor includes an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. The electrolyte impregnated in the separator functions as the cathode in continuity with the cathode foil, while an oxide layer on the anode foil functions as the dielectric.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor increases with the surface area of its electrodes, increasing the surface area of the aluminum anode foil results in increased capacitance per unit volume of the electrolytic capacitor. By electrolytically etching aluminum foils, an enlargement of a surface area of the foil will occur. As a result of this enlargement of the surface area, electrolytic capacitors, which are manufactured with the etched foils, can obtain a given capacity with a smaller volume than an electrolytic capacitor, which utilizes a foil with an unetched surface.
In a conventional electrolytic etching process, surface area of the foil is increased by removing portions of the aluminum foil to create etch tunnels. While electrolytic capacitors having anodes and cathodes comprised of aluminum foil are most common, anode and cathode foils of other conventional valve metals such as titanium, tantalum, magnesium, niobium, zirconium and zinc are also used.
U.S. Pat. No. 4,213,835 to Fickelscher discloses a method for electrolytically etching an aluminum foil. This method involves a pari-potentiostatic etching technique using a constant anode potential in a traveling neutral pH bath containing chloride ions. Foils are thus provided with tunnel densities greater than 107 tunnels/cm2 of foil surface.
U.S. Pat. No. 4,420,367 to Locher discloses a similar method for etching aluminum foil for electrolytic capacitors. In a first etching step, the tunnels are formed electrolytically, as described above, using a neutral pH etching solution. In a second etching step, the tunnels are enlarged using a non-electrolytic process, which involves one or more chemical etching stages.
U.S. Pat. Nos. 4,474,657, 4,518,471 and 4,525,249 to Arora disclose the etching of aluminum electrolytic capacitor foil by passing the foil through an electrolyte bath. The preferred bath contains 3% hydrochloric acid and 1% aluminum as aluminum chloride. The etching is carried out under a direct current (DC) and at a temperature of 75° C. U.S. Pat. No. 4,518,471 adds a second step where the etched foil is treated in a similar bath with a lower current density and at a temperature of 80-82.5° C. U.S. Pat. No. 4,525,249 adds a different second step, where the etched foil is treated in a bath of 8% nitric acid and 2.6% aluminum as a nitrate, at a temperature of 85° C.
U.S. Pat No. 5,901,032 to Harrington, describes a neutral etching solution that consists of sodium chloride, sodium perchlorate, and sodium persulfate.
Typically, the anode foil element in an ICD is comprised of a single or double layer of etched foil. U.S. Pat. No. 5,594,890 to MacFarlane and Lunsmann describes a double layer or greater etched foil. While this multiple anode stack configuration allows for higher energy storage densities, the drawback is that the Equivalent Series Resistance (ESR) of the capacitor increases in the multilayer design. An etching process is needed that produces anode foils of high capacitance, but also, one that produces foils that when stacked in a multilayer design, lead to capacitors with reduced ESR.